Process for producing cermet cutting tools having both longitudinal and granular crystal structures

ABSTRACT

A cermet cutting tool, and process for producing the same, comprising a substrate formed from titanium carbo-nitride based cermet, and a hard coating layer of average thickness 0.5˜20 μm, formed onto the surface of the cermet substrate comprising a lower layer formed from at least one layer of a compound selected from the group consisting of titanium carbide (TiC), titanium nitride (TiN), titanium carbo-nitride (TiCN), titanium carbo-oxide (TiCO) and titanium carbo-oxi-nitride (TiCNO), and aluminum oxide (Al 2  O 3 ). Additionally, at least one of the layers comprising the hard coating layer is a titanium carbo-nitride layer, and at least one layer of this titanium carbo-nitride layer comprises a longitudinal growth crystal structure, and a further layer comprises a granular crystal structure. In the method of fabrication, chemical vapor deposition is performed using a reaction gas composed of 1˜5% of TiCl 4 , 0.1˜1% of CH 3  CN, 0˜25% of N 2  with the remaining portion being composed of H 2 , under a reaction temperature of 800°˜900° C. and a reaction pressure of 30˜200 Torr.

This is a divisional of application Ser. No. 08/073,328, filed on Jun.8, 1993, now U.S. Pat. No. 5,436,071, which is a CIP of Ser. No.07/644,446, filed Jan. 23, 1991, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is a continuation-in-part application of U.S.patent application Ser. No. 644,446, now abandoned, and relates to acermet cutting tool, and process for producing the same, in which a hardcoating layer is formed onto the surface of a titanium carbo-nitridebased cermet substrate. In particular, the present invention pertains toimprovements for enhancing the chipping resistance and wear resistanceof the blade member.

2. Background Art

Conventionally, cermets, containing at least one metal of carbide,nitride and carbo-nitride of titanium (Ti), zirconium (Zr), hafnium(Hf), tantalum (Ta), niobium (Nb), tungsten (W), molybdenum (Mo) andchromium (Cr) as a hard phase-constituting component and iron familymetals such as cobalt (Co), nickel (Ni) and the like as binder metals,have been widely used to form blade members or inserts for use in finishcutting of steel.

In recent years, the aforementioned cermets possessing surface coatingscomposed of TiC, TiCN and the like, which further improve wearresistance, are being produced.

For example, Japanese Patent Application First Publication No. Sho54-117510 discloses a cermet cutting tool which is formed by applying toa titanium carbo-nitride based cermet substrate using a chemical vapordeposition process, a hard coating layer of thickness 0.5˜20 μm which isformed from at least one layer of a compound selected from the groupconsisting of titanium carbide (TiC), titanium nitride (TiN), titaniumcarbo-nitride (TiCN), titanium carbo-oxide (TiCO), titaniumcarbo-oxi-nitride (TiCNO) and aluminum oxide (Al₂ O₃). In addition, asthe substrate, use of a TiCN based cermet in which the incorporatedamount of binding phase-constituting component, Co or Ni, iscomparatively higher in the surface layer portion when compared to thesubstrate interior, is known.

In this case, as the chemical vapor deposition process, a conventionalprocess is employed in which a TiCN coating layer is formed onto thecermet surface using a mixed gas composed of 4% of TiCl₄, 3% of CH₄, 4%of N₂, and 89% of H₂ (% by volume) at a temperature of 1000° C. and apressure of 100 Torr inside the chemical vapor deposition apparatus. Thereaction in this case is represented by the following formula:

    TiCl.sub.4 +CH.sub.4 + 1/2N.sub.2 +H.sub.2 →TiCN+4HCl+H.sub.2

However, when the aforementioned prior art cermet cutting tools are usedin steel and cast-iron high-speed cutting inserts, chipping of thecutting insert occurs easily due to the lack of toughness of the hardcoating layer, and considerable wear occurs due to this chipping,thereby exhausting the tool life in a comparatively short time.

One source generating the aforementioned problems lies in the fact thatthe binding phase-constituting component (Ni or Co) diffuses into theTiCN layer during chemical vapor deposition, creating a materiallyinferior TiCN layer. In other words, the iron family metals such as Coand Ni which comprise the binding phase-constituting components, due tothe high coating temperature (1000° C.) of prior art chemical vapordeposition processes, diffuse into the TiCN layer and cause loss of theoriginal wear resistance, resulting in formation of a coating layerpossessing a low wear resistance. However, if chemical vapor depositionis attempted at a temperature of 900° C. or less where it is difficultfor the binding phase-constituting component to diffuse into the TiCNlayer, the activity of reaction gases such as methane, nitrogen and thelike is reduced, and the reaction does not occur, which results in theTiCN coating layer being unobtainable. Consequently, under theaforementioned temperature conditions of the prior art, a coating layercannot be formed.

The inventors of the present invention have conducted various researchin order to obtain a TiCN coating layer with superior wear resistance byefficient chemical vapor deposition of a TiCN layer at a temperature of900° C. or less, where it is difficult for the aforementioned bindingphase-constituting component to diffuse into the TiCN layer. As aresult, when adding acetonitrile (CH₃ CN) to the mixed gas used in thechemical vapor deposition, highly efficient formation of a TiCN coatinglayer is possible even when chemical vapor deposition is performed at atemperature of 900° C. or less: the TiCN coating layer formed in thismanner, due to non-diffusion of the aforementioned bindingphase-constituting component, was confirmed by observation to possess asuperior wear resistance.

Furthermore, the inventors of the present invention, in order to furtherimprove the wear resistance of the TiCN layer, have continued theirresearch. As a result, they have discovered a novel development in thatwhen chemical vapor deposition is performed using a reaction gascomposed of 1˜5% of TiCl₄, 0.1˜1% of CH₃ CN, 0˜25% of N₂ with theremaining portion being composed of H₂, under a reaction temperature of800°˜900° C. and a reaction pressure of 30˜200 Torr, at least oneportion of the TiCN layer comprises a longitudinal growth crystalstructure, which is a change from the normal granular crystal structure,resulting in further improvement of both the chipping resistance and thewear resistance.

SUMMARY OF THE INVENTION

The present invention is based on this aforementioned discovery, and itis therefore an object of the present invention to provide, by means ofimproving the toughness of the hard coating layer, a cermet cuttingtool, and process for producing the same, with improved chippingresistance and wear resistance of the cutting insert.

In order to achieve this objective, a first aspect of the presentinvention is directed to providing a cermet cutting tool comprising thefollowing:

a. a substrate formed from titanium carbo-nitride based cermet; and

b. a hard coating layer of average thickness 0.5˜20 μm, formed onto thesurface of the aforementioned substrate; this layer being formed from atleast one layer of a compound selected from the group consisting oftitanium carbide (TiC), titanium nitride (TiN), titanium carbo-nitride(TiCN), titanium carbo-oxide (TiCO), titanium carbo-oxi-nitride (TiCNO)and aluminum oxide (Al₂ O₃); wherein at least one of the layerscomprising the aforementioned hard coating layer is a titaniumcarbo-nitride layer, and at least one layer of this titaniumcarbo-nitride layer comprises a longitudinal growth crystal structure.

In addition, a second aspect of the present invention is directed toproviding a cermet cutting tool comprising the following:

a. a substrate formed from titanium carbo-nitride based cermet; and

b. a hard coating layer of average thickness 0.5˜20 μm, formed onto thesurface of the aforementioned substrate comprising at least one layer ofthe compound titanium carbo-nitride; the aforementioned at least onelayer of the compound titanium carbo-nitride comprising theabove-mentioned hard coating layer comprises at least one of thefollowing crystal structures:

(1) a crystal structure which changes from a granular crystal structureto a longitudinal growth crystal structure as it progresses away fromthe aforementioned substrate;

(2) a crystal structure which changes from a granular crystal structureto a longitudinal growth crystal structure and back to a granularcrystal structure as it progresses away from the aforementionedsubstrate; and

(3) a crystal structure which changes from a longitudinal growth crystalstructure to a granular crystal structure as it progresses away from theaforementioned substrate.

Furthermore, a third aspect of the present invention is directed toproviding a cermet cutting tool comprising the following:

a. a substrate formed from titanium carbo-nitride based cermet; and

b. a hard coating layer of average thickness 0.5˜20 μm, formed onto thesurface of the aforementioned substrate comprising a lower layercomposed of at least one layer of the compound titanium carbo-nitride,and an upper layer formed from at least one layer of a compound selectedfrom the group consisting of titanium carbide (TiC), titanium nitride(TiN), titanium carbo-nitride (TiCN), titanium carbo-oxide (TiCO),titanium carbo-oxi-nitride (TiCNO) and aluminum oxide (Al₂ O₃); theentire aforementioned lower layer comprises at least one of thefollowing crystal structures:

(1) a crystal structure which changes from a granular crystal structureto a longitudinal growth crystal structure as it progresses away fromthe aforementioned substrate;

(2) a crystal structure which changes from a granular crystal structureto a longitudinal growth crystal structure and back to a granularcrystal structure as it progresses away from the aforementionedsubstrate; and

(3) a crystal structure which changes from a longitudinal growth crystalstructure to a granular crystal structure as it progresses away from theaforementioned substrate.

A fourth aspect of the present invention is directed to providing acermet cutting tool comprising the following:

a. a substrate formed from titanium carbo-nitride based cermet; and

b. a hard coating layer of average thickness 0.5˜20 μm, formed onto thesurface of the aforementioned substrate comprising a lower layer formedfrom at least one layer of a compound selected from the group consistingof titanium carbide (TiC), titanium nitride (TiN), titaniumcarbo-nitride (TiCN), titanium carbo-oxide (TiCO) and titaniumcarbo-oxi-nitride (TiCNO), and an upper layer formed from at least onelayer of a compound selected from the group consisting of titaniumcarbide (TiC), titanium nitride (TiN), titanium carbo-nitride (TiCN),titanium carbo-oxide (TiCO), titanium carbo-oxi-nitride (TiCNO) andaluminum oxide (Al₂ O₃); wherein at least one of the layers comprisingthe aforementioned upper layer is a titanium carbo-nitride layer, and atleast one layer of this titanium carbo-nitride layer comprises at leastone of the following crystal structures:

(1) a crystal structure which changes from a granular crystal structureto a longitudinal growth crystal structure as it progresses away fromthe aforementioned substrate;

(2) a crystal structure which changes from a granular crystal structureto a longitudinal growth crystal structure and back to a granularcrystal structure as it progresses away from the aforementionedsubstrate; and

(3) a crystal structure which changes from a longitudinal growth crystalstructure to a granular crystal structure as it progresses away from theaforementioned substrate.

In all of the cermet cutting tools provided in aforementioned firstthrough fourth aspects of the present invention, if the averagethickness of the hard coating layer is less than 0.5 μm, sufficient wearresistance is unobtainable due to this hard coating layer. On the otherhand, if the average thickness of the hard coating layer exceeds 20 μm,chipping of the cutting insert occurs easily and a superior wearresistance becomes impossible to obtain.

The cermet comprising the aforementioned substrate contains 70˜95% byweight of a solid solution of carbo-nitride represented by the formula(Ti_(x) M_(y))(C_(u) N_(v)), (wherein M is at least one metal selectedfrom the group consisting of transition metals in groups IV_(A), V_(A)and VI_(A) of the Periodic Table with the exception of titanium, and x,y, u and v are molar ratios which satisfy the relations of x+y=1, u+v=1,0.5≦x≦0.95, 0.05≦y≦0.5, 0.2≦u≦0.8 and 0.2≦v≦0.8) as the hard-dispersedphase-constituting component, with the remainder comprising one or twometals selected from the group consisting of cobalt and nickel as abinder phase-constituting component.

If the content of the aforementioned hard-dispersed phase-constitutingcomponent is less than 70% by weight and the content of the binderphase-constituting component exceeds 30% by weight, the wear resistanceof the cermet substrate is lowered. On the other hand, if the content ofhard-dispersed phase-constituting component exceeds 95% by weight andthe content of the binder phase-constituting component is less than 5%by weight, the cermet substrate becomes susceptible to chipping, inother words, the chipping resistance is poor, both of which areundesirable results.

It is possible for the incorporated amount of the aforementioned binderphase-constituting component to be relatively greater in the surfaceportion of the aforementioned cermet when compared with that of thesubstrate interior. In the case when the incorporated amount of thebinder phase-constituting component is greater at the substrate surface,the chipping resistance of the cermet cutting tool is further improved.

Each of the cermet cutting tools of the present invention having theaforementioned respective structures, as a result of possessing a hardcoating layer with a high toughness at least one portion of whichcomprises a TiCN layer with a longitudinal growth crystal structure,displayed a superior wear resistance when used in steel and cast-ironhigh-speed cutting inserts which, in particular, demand a highresistance to chipping. Consequently, superior cutting properties can bemaintained over a long period of time.

In addition, the process for producing the cermet cutting tool of thepresent invention comprises the following steps:

a. a step for preparing a substrate from a titanium carbo-nitride basedcermet;

b. a step for forming a hard coating layer onto the surface of theaforementioned substrate using a chemical vapor deposition methodwherein in at least one part of this step, chemical vapor deposition isperformed using a reaction gas composed of 1˜5% of TiCl₄, 0.1˜1% of CH₃CN, 0˜25% of N₂ with the remaining portion being composed of H₂, under areaction temperature of 800°˜900° C. and a reaction pressure of 30˜200Torr.

In this process, the reaction in which TiCN is formed is represented bythe following formula:

    TiCl.sub.4 +CH.sub.3 CN+ 5/2H.sub.2 →TiCN+4HCl+CH.sub.4

By means of the aforementioned process, a hard coating layerincorporating a TiCN layer with at least one portion containinglongitudinal growth crystal structure can be formed, and the chippingand wear resistances of this hard coating layer can be improved. Inaddition, in accordance with this process, diffusion of the bindingphase-constituting component of the titanium carbo-nitride based cermetinto the TiCN layer is extremely low, thus reduction of the materialquality of the TiCN layer stemming from diffusion of the bindingphase-constituting component can be prevented, resulting in theenhancement of both chipping and wear resistances.

However, if the aforementioned reaction temperature is less than 800°C., it becomes impossible to form a TiCN layer with longitudinal growthcrystal structure. However, if this reaction temperature exceeds 900°C., the binding phase-constituting component diffuses into the TiCNcoating layer resulting in formation of a coating layer with low wearresistance, both of which are undesirable. In addition, if the reactionpressure is less than 30 Torr, the reaction time is too long, which isan industrially undesirable result. On the other hand, if the reactionpressure exceeds 200 Torr, the reaction speed is too fast resulting in anon-uniform membrane thickness. In addition, if any of theconcentrations of the TiCl₄, CH₃ CN, and N₂ components of theaforementioned reaction gas fall outside the above prescribed ranges,formation of a TiCN layer with longitudinal growth crystal structurebecomes difficult.

It is also possible to form a TiN layer above and/or below the TiCNlayer. As the method for forming this TiN layer, conventional methodsknown in the prior art in which chemical vapor deposition is performedusing nitrogen gas or ammonium gas may be employed. In forming the TiNlayer, a reaction temperature of 900° C. or greater is required. Inaddition, the wettability of the TiN layer with the bindingphase-constituting component of the cermet substrate (ie. Co or Ni) islow, which results in greater prevention of diffusion of the bindingphase-constituting component into the TiCN layer. In this case, themembrane thickness of the TiN layer is preferably less than 4 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged photograph displaying a cross-sectional view ofthe surface portion of a cermet cutting tool (cutting tool 2 of thepresent invention) according to the present invention taken by scanningelectron microscope.

FIG. 2 is an enlarged photograph displaying a cross-sectional view ofthe surface portion of a prior art cermet cutting tool (prior artcutting tool 2) taken by scanning electron microscope.

FIG. 3 is an enlarged photograph displaying a cross-sectional view ofthe surface portion of a cermet cutting tool (cutting tool 13 of thepresent invention) according to the present invention taken by scanningelectron microscope.

FIG. 4 is an enlarged photograph displaying a cross-sectional view ofthe surface portion of a prior art cermet cutting tool (prior artcutting tool 15) taken by scanning electron microscope.

FIG. 5 is an enlarged photograph displaying a cross-sectional view ofthe surface portion of a cermet cutting tool (cutting tool 31 of thepresent invention) according to the present invention taken by scanningelectron microscope.

FIG. 6 is an enlarged photograph displaying a cross-sectional view ofthe surface portion of a prior art cermet cutting tool (prior artcutting tool 26) taken by scanning electron microscope.

FIG. 7 is an enlarged photograph displaying a cross-sectional view ofthe surface portion of a cermet cutting tool (cutting tool 38 of thepresent invention) according to the present invention taken by scanningelectron microscope.

FIG. 8 is an enlarged photograph displaying a cross-sectional view ofthe surface portion of a prior art cermet cutting tool (prior artcutting tool 27) taken by scanning electron microscope.

EXAMPLES

The present invention will now be explained with reference to theExamples.

Example 1

Powders of TiCN, TiC, TiN, TaC, NbC, WC, Mo₂ C, ZrC, Co and Ni eachhaving a predetermined average particle size within the range of 1˜3 μm,were prepared as starting materials. These starting materials werecombined so as to produce the composition shown in TABLE 1, and weremixed together for 72 hours using a ball mill under wet conditions.After drying, the resultant mixed powder was pressed and a large numberof green compact bodies were formed.

A portion of these compacted bodies were then sintered at a fixedtemperature from 1450°˜1500° C. for one hour under 10⁻² Torr in a vacuumto produce TiCN based cermet substrates A˜D in which the bindingphase-constituting component was not contained in a relatively largeramount at the surface layer portion.

The remainder of the aforementioned compacted bodies were first heatedunder rising temperature and brought to sintering temperature under 10⁻²Torr in a vacuum; after reaching a predetermined temperature within therange of 1450°˜1550° C., the sintering conditions were changed to 10Torr in N₂ environment and maintained for one hour; and cooling fromsintering temperature was performed in a vacuum under 0.1 Torr toproduce TiCN based cermet substrates E and F in which the bindingphase-constituting component was contained in a relatively larger amountat the surface layer portion. The respective thicknesses of the surfacelayer portions of these substrates E and F, and the maximum amount ofbinding phase-constituting component incorporated into these surfacelayer portions are shown in TABLE 1.

Furthermore, all of the aforementioned TiCN based cermet substrates A˜Fwere shaped to conform to cutting chip standards of ISO-CNMG120408.

Each respective type of coating layer comprising granular crystalstructures, and TiCN layers comprising longitudinal growth crystalstructures were formed onto the surfaces of the aforementioned TiCNbased cermet substrates A˜F using a conventional chemical vapordeposition apparatus, according to the layer constructions shown inTABLES 3 and 4, to produce cutting tools 1˜12 of the present invention.The coating layers comprising granular crystal structures were formedunder the conditions shown in TABLE 2, while the TiCN layers comprisinglongitudinal growth crystal structures were formed under the followingconditions.

Conditions for Forming Longitudinal Growth Crystal Structure

Reaction gas composition:

TiCl₄ --1.5% by volume

CH₃ CN--0.5% by volume

N₂ --25% by volume

H₂ --73% by volume

Reaction temperature: 860° C.

Pressure: 50 Torr

On the other hand, under the conditions shown in TABLE 2, the hardcoating layers, comprising granular crystal structures and having therespective average thicknesses and compositions shown in TABLES 4 and 5,were formed onto the surfaces of the aforementioned TiCN based cermetsubstrates A˜F to produce prior art TiCN based cermet cutting tools1˜12.

Subsequently, all of the cermet cutting tools were subjected to both asteel wet-type continuous hitch-speed cutting test and a steel dry-typeintermittent high-speed cutting test. The continuous cutting testmeasured the amount of flank wear of the cutting edge, while theintermittent cutting test measured the amount of time necessary to reachthe limits of tool life with chipping as the source. The results ofthese tests are shown in TABLE 6, with the cutting test conditions asfollows:

Continuous cutting test conditions

Workpiece: Round bar (JIS. SCM440)

Cutting speed: 250 m/min.

Feed rate: 0.2 mm/rev.

Depth of cut: 1 mm

Cutting time: 30 min.

Intermittent Cutting Test Conditions

Workpiece: Round bar with 4 longitudinal grooves (JIS. SNCM439)

Cutting speed: 200 m/min.

Feed rate: 0.2 mm/rev.

Depth of cut: 1 mm

In addition, photographs enlarged 5000 times displaying the longitudinalcross-sectional structures of the surface portions of cutting tool 2 ofthe present invention and cutting tool 2 of the prior art, as taken byscanning electron microscope, are shown in FIGS. 1 and 2.

As seen from the results of TABLES 1˜6 and FIGS. 1˜2, cermet cuttingtools 1˜12 of the present invention possessing at least one TiCN layerin their hard coating layer, and at least one layer within this TiCNlayer comprising a longitudinal growth crystal structure, displaysuperior wear resistance in steel high-speed continuous cutting andsuperior chipping resistance in high-speed intermittent cutting, andconsequently display superior cutting properties over a long period oftime when compared with prior art cutting tools 1˜12, all of which areformed with a hard coating layer consisting of granular crystalstructure.

Example 2

Powders of TiCN, TiC, TiN, TaC, NbC, WC, Mo₂ C, ZrC, Co and Ni eachhaving a predetermined average particle size within the range of 1˜2 μm,were prepared as starting materials. These starting materials werecombined so as to produce the compositions shown in TABLE 7, andprocessed under the same conditions as in Example 1 to produce TiCNbased cermet substrates G˜I, in which the binding phase-constitutingcomponent was not contained in a relatively larger amount at the surfacelayer portion, and TiCN based cermet substrates J˜L, in which thebinding phase-constituting component was contained in a relativelylarger amount at the surface layer portion. The thicknesses of thesurface layer portions of the substrates containing the bindingphase-constituting component in a relatively larger amount, and themaximum amount of binding phase-constituting component incorporated intothese surface layer portions are shown together in TABLE 7.

Subsequently, TiCN layers comprising granular crystal structures, andTiCN layers comprising longitudinal growth crystal structures wereformed onto the surfaces of the aforementioned TiCN based cermetsubstrates G˜L using a conventional chemical vapor deposition apparatusunder the following conditions, according to the respective layerconstructions and thicknesses shown in TABLES 8 and 9, to producecutting tools 13˜24 of the present invention.

Conditions for Forming TiCN Granular Crystal Structure

Reaction gas composition:

TiCl₄ --3% by volume

CH₄ --5% by volume

N₂ --25% by volume

H₂ --67% by volume

Reaction temperature: 1020° C.

Pressure: 100 Torr

Conditions for Forming Longitudinal Growth Crystal Structure

Reaction gas composition:

TiCl₄ --1.5% by volume

CH₃ CN--0.5% by volume

N₂ --25% by volume

H₂ --73% by volume

Reaction temperature: 860° C.

Pressure: 50 Torr

On the other hand, TiCN layers all comprising granular crystalstructures were formed onto the surfaces of the aforementioned TiCNbased cermet substrates G˜L, as shown in TABLE 9, under exactly the sameconditions as the aforementioned products of the present invention toproduce prior art TiCN based cermet cutting tools 13˜18.

Subsequently, all of the cermet cutting tools were subjected to both asteel dry-type continuous high-speed cutting test and a cast-ironwet-type high-speed cutting test under the conditions outlined below,and the amount of flank wear of the cutting edge was measured. Theresults of these tests are shown in TABLES 8 and 9.

Steel Dry-Type High-Speed Cutting Test Conditions

Workpiece: SCM440; Hardness: H_(B) 220

Cutting speed: 250 m/min.

Feed rate: 0.2 mm/rev.

Depth of cut: 1 mm

Cutting time: 30 min.

Cast-Iron Wet-Type High-Speed Cutting Test Conditions

Workpiece: FC30; Hardness: H_(B) 200

Cutting speed: 300 m/min.

Feed rate: 0.2 mm/rev.

Depth of cut: 1 mm

Cutting time: 30 min.

In addition, photographs enlarged 5000 times displaying the longitudinalcross-sectional structures of the surface portions of cutting tool 13 ofthe present invention and cutting tool 15 of the prior art, as taken byscanning electron microscope, are shown in FIGS. 3 and 4. As shown inFIG. 3, cutting tool 13 of the present invention is formed with acrystal structure which changes from a granular crystal structure to alongitudinal growth crystal structure and then back to a granularcrystal structure as it progresses away from the substrate. In contrast,prior art cutting tool 15, as shown in FIG. 4, comprises a TiCN layercomprising only a granular crystal structure.

As seen from the results of TABLES 8˜9 and FIGS. 3˜4, cermet cuttingtools 13˜24 of the present invention possessing within a TiCN layercomprising the hard coating layer, at least one layer consisting of acrystal structure which changes from a granular crystal structure to alongitudinal growth crystal structure, a crystal structure which changesfrom a granular crystal structure to a longitudinal growth crystalstructure and then back to a granular crystal structure, or a crystalstructure which changes from a longitudinal growth crystal structure toa granular crystal structure, as it progresses away from the substrate,display superior wear resistance and chipping resistance in steel andcast-iron high-speed cutting where chipping occurs easily, when comparedwith prior art cutting tools 13˜18.

Example 3

TiCN based cermet substrates G˜I, in which the bindingphase-constituting component was not contained in a relatively largeramount at the surface layer portion, and TiCN based cermet substratesJ˜L, in which the binding phase-constituting component was contained ina relatively larger amount at the surface layer portion were producedusing the same composition and process as with the cermet substrates ofExample 2.

Subsequently, TiCN layers consisting of granular crystal structures, andTiCN layers consisting of longitudinal growth crystal structures wereformed onto the surfaces of the aforementioned TiCN based cermetsubstrates G˜L using a conventional chemical vapor deposition apparatus,under the same conditions as in Example 2, to produce a TiCN lowerlayer. Furthermore, on top of this lower layer an upper layer,comprising a granular crystal structure and possessing the compositionsand thicknesses shown in TABLE 10, was formed using a conventionalchemical vapor deposition apparatus under the general conditions shownin TABLE 11 to produce cermet cutting tools 25˜32 of the presentinvention.

On the other hand, hard coating layers, comprising granular crystalstructures and possessing the respective compositions and thicknessesshown in TABLE 12, were formed onto the surfaces of the aforementionedTiCN based cermet substrates G˜L, under the general conditions shown inTABLE 11, to produce prior art TiCN based cermet cutting tools 19˜26.

Subsequently, all of the cermet cutting tools were subjected to both asteel dry-type high-speed cutting test and an cast-iron wet-typehigh-speed cutting test as in Example 2, and the amount of flank wear ofthe cutting edge was measured. The results of these tests are shown inTABLE 13.

In addition, photographs enlarged 5000 times displaying the longitudinalcross-sectional structures of the surface portions of cutting tool 31 ofthe present invention and cutting tool 25 of the prior art, as taken byscanning electron microscope, are shown in FIGS. 5 and 6. As shown inFIG. 5, cutting tool 31 of the present invention is formed with a hardcoating layer comprising a lower TiCN layer which changes from agranular crystal structure to a longitudinal growth crystal structure asit progresses away from the substrate, and an upper TiCN layercomprising a granular crystal structure. In contrast, prior art cuttingtool 25, as shown in FIG. 6, comprises TiCN and TiN layers formed withonly a granular crystal structure.

As seen from the results of TABLES 10˜13 and FIGS. 5˜6, cermet cuttingtools 25˜32 of the present invention possessing a hard coating layercomprising a TiCN layer in which at least one layer comprises a crystalstructure which changes from a granular crystal structure to alongitudinal growth crystal structure, a crystal structure which changesfrom a granular crystal structure to a longitudinal growth crystalstructure and then back to a granular crystal structure, or a crystalstructure which changes from a longitudinal growth crystal structure toa granular crystal structure, as it progresses away from the substrate,display superior wear resistance and chipping resistance in steel andcast-iron high-speed cutting where chipping occurs easily, when comparedwith prior art cutting tools 19˜26.

Example 4

TiCN based cermet substrates G˜I, in which the bindingphase-constituting component was not contained in a relatively largeramount at the surface layer portion, and TiCN based cermet substratesJ˜L, in which the binding phase-constituting component was contained ina relatively larger amount at the surface layer portion, were producedusing the same composition and process as with the cermet substrates ofExample 2.

Subsequently, onto each surface of the aforementioned TiCN based cermetsubstrates G˜L, a lower layer comprising at least one layer consistingof a granular crystal structure and possessing the respectivecompositions and thicknesses shown in TABLE 14, was formed using aconventional chemical vapor deposition apparatus under the conditionsshown in TABLE 11.

Following this, under the same conditions as shown in TABLE 11, theupper layers, each comprising at least one layer and possessing therespective compositions and thicknesses shown in TABLE 14, were formed.These upper layers each comprise at least one TiCN layer, and at leastone layer within each respective TiCN layer was combined under thegranular crystal structure forming conditions or longitudinal growthcrystal structure forming conditions described below, to produce cermetcutting tools 33˜40 of the present invention possessing the crystalstructures shown in TABLE 14.

Conditions for Forming TiCN Granular Crystal Structure

Reaction gas composition:

TiCl₄ --3% by volume

CH₄ --5% by volume

N₂ --25% by volume

H₂ --67% by volume

Reaction temperature: 950° C.

Pressure: 100 Torr

Conditions for Forming Longitudinal Growth Crystal Structure

Reaction gas composition:

TiCl₄ --1.5% by volume

CH₃ CN--0.5% by volume

N₂ --25% by volume

H₂ --73% by volume

Reaction temperature: 860° C.

Pressure: 50 Torr

On the other hand, hard coating layers, comprising granular crystalstructures and possessing the respective compositions and thicknessesshown in TABLE 15, were formed onto the surfaces of the aforementionedTiCN based cermet substrates G˜L, under the general conditions shown inTABLE 11, to produce prior art TiCN based cermet cutting tools 27˜34.

Subsequently, all of the cermet cutting tools were subjected to both asteel dry-type high-speed cutting test and an cast-iron wet-typehigh-speed cutting test as in Example 2, and the amount of flank wear ofthe cutting edge was measured. The results of these tests are shown inTABLE 16.

In addition, photographs enlarged 5000 times displaying the longitudinalcross-sectional structures of the surface portions of cutting tool 38 ofthe present invention and cutting tool 32 of the prior art, as taken byscanning electron microscope, are shown in FIGS. 7 and 8.

As seen from the results of TABLES 14˜16 and FIGS. 7˜8, cermet cuttingtools 33˜40 of the present invention, comprising as the upper layer ofthe hard coating layer, at least one TiCN layer comprising a crystalstructure which changes from a granular crystal structure to alongitudinal growth crystal structure, a crystal structure which changesfrom a granular crystal structure to a longitudinal growth crystalstructure and then back to a granular crystal structure, or a crystalstructure which changes from a longitudinal growth crystal structure toa granular crystal structure, as it progresses away from the substrate,display superior wear resistance and chipping resistance in steel andcast-iron high-speed cutting where chipping occurs easily, when comparedwith prior art cutting tools 27˜34.

                                      TABLE 1                                     __________________________________________________________________________                                       Binding phase                              Cermet                             enhanced surface portion                   substrate                          Mix incorporated                                                                       Thick-                            classifi-                                                                          Distributional Composition (% by weight)                                                                    amt. of binding                                                                        ness                              cation                                                                             Co                                                                              Ni                                                                              TaC                                                                              NbC                                                                              WC Mo.sub.2 C                                                                        ZrC                                                                              TiC                                                                              TiN                                                                              TiCN                                                                              phase (% by wt)                                                                        (μm)                           __________________________________________________________________________    A    8 4  9 1  15 8   0.5                                                                              R  20 --  --     --                                  B    8 6 10 1  10 8   -- --  5     --     --                                  C    12                                                                              6  9 1  15 9   0.5                                                                              R  20 --  --     --                                  D    14                                                                              4 15 2  16 --  0.3                                                                              R  20     --     --                                  E    12                                                                              4 10 1  15 --  0.5                                                                              18 R  --  26     20                                  F    7 7 12 1  16 --  0.2                                                                              --  5 R   21     30                                  __________________________________________________________________________     R = Remaining portion                                                    

                                      TABLE 2                                     __________________________________________________________________________    Hard cover-                                                                          Conditions for forming the hard covering layer granular crystal               structure                                                              ing layer                  Reaction environment                               composition                                                                          Reaction gas composition (% by volume)                                                            Pressure (Torr)                                                                       Temp (°C.)                          __________________________________________________________________________    TiC    TiCl.sub.4 : 4%, CH.sub.4 : 5%, H.sub.2 : 91%                                                     100     1030                                       TiN    TiCl.sub.4 : 4%, N.sub.2 : 8%, H.sub.2 : 88%                                                      100      980                                       TiCN   TiCl.sub.4 : 4%, CH.sub.4 : 3%, N.sub.2 : 4%, H.sub.2 :                                           100     1000                                       TiCO   TiCl.sub.4 : 4%, CO: 6%, H.sub.2 : 90%                                                            100     1000                                       TiCNO  TiCl.sub.4 : 4%, CO: 3%, N.sub.2 : 3%, H.sub.2 :                                                  100     1000                                       Al.sub.2 O.sub.3                                                                     AlCl.sub.3 : 3%, CO.sub.2 : 5%, H.sub.2 : 92%                                                     100     1000                                       __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________              Hard covering layer                                                 Classi-   First Layer                                                                             Second Layer                                              fication        Aver-     Aver-                                               (Cutting                                                                            Sub-      age       age Third Layer                                                                              Fourth Layer                                                                             Fifth Layer               tool of                                                                             strate                                                                            Com-  thick-                                                                            Com-  thick                                                                             Com-  Average                                                                            Com-  Average                                                                            Com-  Average             the present                                                                         num-                                                                              position                                                                            ness                                                                              position                                                                            ness                                                                              position                                                                            thickness                                                                          position                                                                            thickness                                                                          position                                                                            thickness           invention)                                                                          ber (structure)                                                                         (μm)                                                                           (structure)                                                                         (μm)                                                                           (structure)                                                                         (μm)                                                                            (structure)                                                                         (μm)                                                                            (structure)                                                                         (μm)             __________________________________________________________________________    1     A   TiN (P)                                                                             1   TiCN (L)                                                                            6   TiN (P)                                                                             0.5  --    --   --    --                  2         TiC (P)                                                                             1   TiCN (L)                                                                            6   --    --   --    --   --    --                  3     B   TiN (P)                                                                             1   TiCN (L)                                                                            6   TiN (P)                                                                             1    --    --   --    --                  4         TiC (P)                                                                             1   TiCN (P)                                                                            1   TiCN (L)                                                                            5    TiCN (P)                                                                            1    TiN                                                                                 1P)                 5     C   TiC (P)                                                                             1   TiCN (L)                                                                            4   TiN (P)                                                                             0.5  --    --   --    --                  6         TiC (P)                                                                             1   TiCN (L)                                                                            4   TiCO (P)                                                                            0.5  Al.sub.2 O.sub.3                                                                    1.5  TiN                                                                                 0.5                 7     D   TiN (P)                                                                             0.5 TiCN (L)                                                                            3   TiN (P)                                                                             0.5  --    --   --    --                  8         TiCN (P)                                                                            1   TiCN (L)                                                                            3   TiCN (P)                                                                            1    TiCN (L)                                                                            2    TiN                                                                                 0.5                 __________________________________________________________________________     P = Granular crystal structure; L = Longitudinal crystal structure       

                                      TABLE 4                                     __________________________________________________________________________            Hard covering layer                                                   Sub-    First Layer                                                                              Second Layer                                                                             Third Layer                                                                              Fourth Layer                                                                             Fifth Layer               Clas-                                                                             strate                                                                            Com-  Average                                                                            Com-  Average                                                                            Com-  Average                                                                            Com-  Average                                                                            Com-  Average             sifi-                                                                             num-                                                                              position                                                                            thickness                                                                          position                                                                            thickness                                                                          position                                                                            thickness                                                                          position                                                                            thickness                                                                          position                                                                            thickness           cation                                                                            ber (structure)                                                                         (μm)                                                                            (structure)                                                                         (μm)                                                                            (structure)                                                                         (μm)                                                                            (structure)                                                                         (μm)                                                                            (structure)                                                                         (μm)             __________________________________________________________________________    Cutting tool covering of                                                      the present invention                                                         9   E   TiN (P)                                                                             1    TiCN (L)                                                                            6    TiN (P)                                                                             0.5  --    --   --    --                  10      TiC (P)                                                                             1    TiCN (L)                                                                            2    TiCN (P)                                                                            1    TiCN (L)                                                                            2    TiN                                                                                 1P)                 11  F   TiN (P)                                                                             1    TiCN (L)                                                                            6    TiCNO (P)                                                                           0.5  Al.sub.2 O.sub.3                                                                    1P)  TiN                                                                                 0.5                 12      TiN (P)                                                                             1    TiCN (L)                                                                            5    --    --   --    --   --    --                  Prior art cutting tool covering                                               1   A   TiN (P)                                                                             1    TiCN (P)                                                                            6    TiN (P)                                                                             0.5  --    --   --   --                   2       TiC (P)                                                                             1    TiCN (P)                                                                            6    --    --   --    --   --   --                   3   B   TiN (P)                                                                             1    TiCN (P)                                                                            6    TiN (P)                                                                             1    --    --   --   --                   4       TiC (P)                                                                             1    TiCN (P)                                                                            7    TiN (P)                                                                             1    --    --   --   --                   __________________________________________________________________________     P = Granular crystal structure; L = Longitudinal crystal structure       

                                      TABLE 5                                     __________________________________________________________________________            Hard covering layer                                                   Sub-    First Layer                                                                              Second Layer                                                                             Third Layer                                                                              Fourth Layer                                                                             Fifth Layer               Clas-                                                                             strate                                                                            Com-  Average                                                                            Com-  Average                                                                            Com-  Average                                                                            Com-  Average                                                                            Com-  Average             sifi-                                                                             num-                                                                              position                                                                            thickness                                                                          position                                                                            thickness                                                                          position                                                                            thickness                                                                          position                                                                            thickness                                                                          position                                                                            thickness           cation                                                                            ber (structure)                                                                         (μm)                                                                            (structure)                                                                         (μm)                                                                            (structure)                                                                         (μm)                                                                            (structure)                                                                         (μm)                                                                            (structure)                                                                         (μm)             __________________________________________________________________________    Prior art cutting tool coverings                                              5   C   TiN (P)                                                                             1    TiCN (P)                                                                            4    TiN (P)                                                                             0.5  --    --   --    --                  6       TiC (P)                                                                             1    TiCN (P)                                                                            4    TiCO (P)                                                                            0.5  Al.sub.2 O.sub.3                                                                    1.5  TiN                                                                                 0.5                 7   D   TiN (P)                                                                             0.5  TiCN (P)                                                                            3    TiN (P)                                                                             0.5  --    --   --    --                  8       TiCN (P)                                                                            7    TiN (P)                                                                             0.5  --    --   --    --   --    --                  9   E   TiN (P)                                                                             1    TiCN (P)                                                                            6    TiN (P)                                                                             0.5  --    --   --    --                  10      TiC (P)                                                                             1    TiCN (P)                                                                            6    TiN (P)                                                                             1    --    --   --    --                  11  F   TiN (P)                                                                             1    TiCN (P)                                                                            6    TiCNO (P)                                                                           0.5  Al.sub.2 O.sub.3                                                                    1P)  TiN                                                                                 0.5                 12      TiN (P)                                                                             1    TiCN (P)                                                                            5    --    --   --    --   --    --                  __________________________________________________________________________     P = Granular crystal structure; L = Longitudinal crystal structure       

                                      TABLE 6                                     __________________________________________________________________________           Continuous                                                                          Intermittent  Continuous                                                                          Intermittent                                        cutting                                                                             cutting       cutting                                                                             cutting                                             Amount of                                                                           Cutting       Amount of                                                                           Cutting                                             flak wear                                                                           time          flak wear                                                                           time                                         Classification                                                                       (mm)  (min.) Classification                                                                       (mm)  (min.)                                       __________________________________________________________________________    Cutting tool covering of the present invention                                Prior art cutting tool covering                                               1      0.11  7.4    1      0.23  1.0                                          2      0.12  7.2    2      0.27  1.2                                          3      0.13  8.0    3      0.26  0.9                                          4      0.12  8.6    4      0.28  1.1                                          5      0.15  12.0   5      0.27  1.4                                          6      0.13  8.5    6      0.27  1.6                                          7      0.22  13.6   7      0.34  2.3                                          8      0.15  10.9   8      0.26  2.0                                          9      0.12  12.5   9      0.22  1.8                                          10     0.14  12.8   10     0.24  2.2                                          11     0.12  9.0    11     0.21  2.1                                          12     0.20  9.7    12     0.38  1.9                                          __________________________________________________________________________

                                      TABLE 7                                     __________________________________________________________________________                                       Binding phase enhanced                                                        surface portion                                                               Mix incorporated                                                              amount of                                                                              Thick-                            Classifi-                                                                          Distributional Composition (% by weight)                                                                    binding phase                                                                          ness                              cation                                                                             Co                                                                              Ni                                                                              TaC                                                                              NbC                                                                              WC Mo.sub.2 C                                                                        ZrC                                                                              TiC                                                                              TiN                                                                              TiCN                                                                              (% by wt)                                                                              (μm)                           __________________________________________________________________________    Cermet substrate                                                              G    4 8 10 1  10 10  -- R  30 --  --       --                                H    12                                                                              6 8  -- 15 10  0.5                                                                              R  20 --  --       --                                I    10                                                                              10                                                                              -- 10 10 6   -- -- 10 R   --       --                                J    7 7 10 1  8  8   -- -- 5  R   20       10                                K    12                                                                              4 12 -- 15 --  -- R  35 --  28       20                                L    13                                                                              5 15 -- 15 --  0.5                                                                              -- -- R   22       15                                __________________________________________________________________________     R = Remaining portion                                                    

                                      TABLE 8                                     __________________________________________________________________________                                                        Amount of flank                                                               wear (mm)                           Crystal structure of the TiCN layer       Cutting                                                                            Cutting              Classifi-                                                                          Substrate                                                                          First layer                                                                            Second layer                                                                          Third layer                                                                           Fourth layer                                                                          Fifth layer                                                                            Material:                                                                          Material:            cation                                                                             number                                                                             (Thickness: μm)                                                                     (Thickness: μm)                                                                    (Thickness: μm)                                                                    (Thickness: μm)                                                                    (Thickness: μm)                                                                     Steel                                                                              Cast-iron            __________________________________________________________________________    Cutting tool covering of the present invention                                13   G    P + L (U) (6)                                                                          --      --      --      --       0.17 0.16                 14        P + L (I) + P                                                                          L (B) + P (2)                                                                         P (1)   --      --       0.16 0.15                           (4)                                                                 15   H    P + L (I) + P                                                                          --      --      --      --       0.14 0.21                           (8)                                                                 16        P + L (U) (3)                                                                          P + L (U) (3)                                                                         P + L (U) (3)                                                                         P + L (U) (3)                                                                         P + L (I) + P                                                                          0.12 0.18                                                            (3)                                17   I    L (B) + P (4)                                                                          --      --      --      --       0.19 0.22                 18        P + L (U) (4)                                                                          P + L (I) + P                                                                         L (B) + P (2)                                                                         --      --       0.15 0.19                                    (3)                                                        19   J    P + L (U) (6)                                                                          P (1)   --      --      --       0.16 0.14                 20        P (1)    L (B) + P (4)                                                                         L (B) + P (3)                                                                         L (B) + P (2)                                                                         --       0.12 0.13                 21   K    P + L (U) (7)                                                                          P + L (U) (1)                                                                         --      --      --       0.14 0.16                 __________________________________________________________________________     Note:                                                                         P = Granular crystal structure; L = Longitudinal growth crystal structure     U = Upper layer; I = Intermediate layer; B = Lower layer                      *Thickness represents the average thickness                              

                                      TABLE 9                                     __________________________________________________________________________                                                      Amount of flank                                                               wear (mm)                             Crystal structure of the TiCN layer     Cutting                                                                             Cutting               Classifi-                                                                          Substrate                                                                          First layer                                                                           Second layer                                                                          Third layer                                                                           Fourth layer                                                                          Fifth layer                                                                           Material:                                                                           Material:             cation                                                                             number                                                                             (Thickness: μm)                                                                    (Thickness: μm)                                                                    (Thickness: μm)                                                                    (Thickness: μm)                                                                    (Thickness: μm)                                                                    Steel Cast-iron             __________________________________________________________________________    Cutting tool covering of the                                                  present invention                                                             22   K    P (0.5) L (B) + P (4)                                                                         P (1)   L (B) + P (4)                                                                         P (0.5) 0.13  0.14                  23   L    P + L (I) + P                                                                         P (1)   --      --      --      0.21  0.23                            (3)                                                                 24        P + L (I) + P                                                                         L (B) + P (3)                                                                         L (B) + P (3)                                                                         --      --      0.14  0.21                            (5)                                                                 Prior art cutting tool coverings                                              13   G    P (6)   --      --      --      --      9 min.                                                                              7 min. tool                                                             life due                                                                            life due to                                                             chipping                                                                            chipping              14   H    P (8)   --      --      --      --      6 min.                                                                              2 min. tool                                                             life due                                                                            life due to                                                             chipping                                                                            chipping              15   I    P (4)   --      --      --      --      11 min.                                                                             9 min. tool                                                             life due                                                                            life due to                                                             chipping                                                                            chipping              16   J    P (10)  --      --      --      --      5 min.                                                                              3 min. tool                                                             life due                                                                            life due to                                                             chipping                                                                            chipping              17   K    P (10)  --      --      --      --      4 min.                                                                              2 min. tool                                                             life due                                                                            life due to                                                             chipping                                                                            chipping              18   L    P (11)  --      --      --      --      6 min.                                                                              5 min. tool                                                             life due                                                                            life due to                                                             chipping                                                                            chipping              __________________________________________________________________________     Note:                                                                         P = Granular crystal structure; L = Longitudinal growth crystal structure     U = Upper layer; I = Intermediate layer; B = Lower layer                      *Thickness represents the average thickness                              

                                      TABLE 10                                    __________________________________________________________________________             Hard covering layer                                                           Crystal structure of the lower layer (TiCN layer)                                                           Composition of the upper layer and                                            average                                Clas-    and average thickness of each layer (μm)                                                                 thickness of each layer (μm)        sifi-                                                                             Substrate                                                                          First   Second  Third Fourth  First  Second Third                                                                              Fourth              cation                                                                            number                                                                             layer   layer   layer layer   layer  layer  layer                                                                              layer               __________________________________________________________________________    Cutting tool covering of the                                                  present invention                                                             25  G    P + L (U)                                                                             --      --    --      TiCO (0.5)                                                                           Al.sub.2 O.sub.3                                                                     --)  --                           (5)                                                                  26       P + L (I) + P                                                                         L (B) + P (2)                                                                         --    --      TiN (0.5)                                                                            --     --   --                           (6)                                                                  27  H    L (B) + P (4)                                                                         --      --    --      TiC (2)                                                                              TiN (1)                                                                              --   --                  28  I    P + L (U) (2)                                                                         P + L (U) (2)                                                                         P + L (U)                                                                           P + L (U) (2)                                                                         TiCNO (0.5)                                                                          Al.sub.2 O.sub.3 (1)                                     (2)                                                  29  J    P + L (I) + P                                                                         --      --    --      TiC (2)                                                                              TiCNO (0.5)                                                                          Al.sub.2 O.sub.3                                                                   TiN (0.5)                    (2)                                                                  30       P + L (U) (2)                                                                         P + L (I) + P                                                                         L (B) + P                                                                           --      TiN (1)                                                                              --     --   --                                   (3)     (2)                                                  31  K    P + L (U) (7)                                                                         --      --    --      TiN (1)                                                                              --     --   --                  32  L    P + L (I) + P                                                                         L (B) + P                                                                             P + L (U)                                                                           P + L (U) (2)                                                                         TiN (0.5)                                                                            --     --   --                           (2.5)   (2)     (2)                                                  __________________________________________________________________________     Note:                                                                         P = Granular crystal structure; L = Longitudinal growth crystal structure     U = Upper layer; I = Intermediate layer; B = Lower layer                 

                                      TABLE 11                                    __________________________________________________________________________    Hard                                                                          covering                                                                            Conditions for forming the hard covering layer granular structure       layer                       Reaction environment                              composi-                    Pressure                                                                           Temp                                         tion  Reaction gas composition (% by volume)                                                              (Torr)                                                                             (°C.)                                 __________________________________________________________________________    TiC   TiCl.sub.4 : 4%, CH.sub.4 : 5%, H.sub.2 : 91%                                                       100  1030                                         TiN   TiCl.sub.4 : 4%, N.sub.2 : 8%, H.sub.2 : 88%                                                             1000                                         TiCN  TiCl.sub.4 : 4%, CH.sub.4 : 3%, N.sub.2 : 4%, H.sub.2 :                                                   980                                         TiCO  TiCl.sub.4 : 4%, CO: 6%, H.sub.2 : 90%                                                                   1000                                         TiCNO TiCl.sub.4 : 4%, CO: 3%, N.sub.2 : 3%, H.sub.2 : 90%                                                     1000                                         Al.sub.2 O.sub.3                                                                    Al.sub.2 O.sub.3 : 3%, CO.sub.2 : 5%, H.sub.2 : 92%                                                      1050                                         __________________________________________________________________________

                                      TABLE 12                                    __________________________________________________________________________              Composition of the hard covering layer and thickness                          of each layer (μm)                                               Classifi-                                                                          Substrate                                                                          First                                                                              Second Third  Fourth                                                                             Fifth                                       cation                                                                             number                                                                             layer                                                                              layer  layer  layer                                                                              layer                                       __________________________________________________________________________    Prior art cutting tool covering                                               19   G    TiCN (5)                                                                           TiCO (0.5)                                                                           Al.sub.2 O.sub.3 (1)                                                                 --   --                                          20        TiCN (8)                                                                           TiN (0.5)                                                                            --     --   --                                          21   H    TiCN (4)                                                                           TiC (2)                                                                              TiN (1)                                                                              --   --                                          22   I    TiCN (8)                                                                           TiCNO (0.5)                                                                          Al.sub.2 O.sub.3 (1)                                                                 --   --                                          23   J    TiCN (2)                                                                           TiC (2)                                                                              TiCNO (0.5)                                                                          Al.sub.2 O.sub.3 (1)                                                               TiN (0.5)                                   24        TiCN (8)                                                                           TiN (1)                                                                              --     --   --                                          25   K    TiCN (7)                                                                           TiN (1)                                                                              --     --   --                                          26   L    TiCN (5)                                                                           TiN (0.5)                                                                            --     --   --                                          __________________________________________________________________________

                  TABLE 13                                                        ______________________________________                                        Amount                                                                        of flank wear            Amount of flank wear                                 (mm)                     (mm)                                                 Clas- Cutting  Cutting         Cutting Cutting                                sifi- Material:                                                                              Material:                                                                              Classifi-                                                                            Material:                                                                             Material:                              cation                                                                              Steel    Cast-iron                                                                              cation Steel   Cast-iron                              ______________________________________                                        Cutting tool covering of                                                                      Prior art cutting                                             the present invention                                                                         tool covering                                                 25    0.15     0.12     19     7 min. tool                                                                           5 min. tool                                                           life due to                                                                           life due to                                                           chipping                                                                              chipping                               26    0.13     0.12     20     6 min. tool                                                                           4 min. tool                                                           life due to                                                                           life due to                                                           chipping                                                                              chipping                               27    0.16     0.14     21     8 min. tool                                                                           8 min. tool                                                           life due to                                                                           life due to                                                           chipping                                                                              chipping                               28    0.18     0.16     22     3 min. tool                                                                           2 min. tool                                                           life due to                                                                           life due to                                                           chipping                                                                              chipping                               29    0.15     0.14     23     9 min. tool                                                                           10 min. tool                                                          life due to                                                                           life due to                                                           chipping                                                                              chipping                               30    0.14     0.16     24     7 min. tool                                                                           4 min. tool                                                           life due to                                                                           life due to                                                           chipping                                                                              chipping                               31    0.13     0.15     25     8 min. tool                                                                           5 min. tool                                                           life due to                                                                           life due to                                                           chipping                                                                              chipping                               32    0.17     0.18     26     6 min. tool                                                                           8 min. tool                                                           life due to                                                                           life due to                                                           chipping                                                                              chipping                               ______________________________________                                    

                                      TABLE 14                                    __________________________________________________________________________             Hard covering layer                                                           Crystal structure of the lower layer (TiCN layer)                                                           Composition of the upper layer and                                            average                                Clas-    and average thickness of each layer (μm)                                                                 thickness of each layer (μm)        sifi-                                                                             Substrate                                                                          First   Second  Third Fourth  First Second  Third                                                                              Fourth              cation                                                                            number                                                                             layer   layer   layer layer   layer layer   layer                                                                              layer               __________________________________________________________________________    Cutting tool covering of                                                      the present invention                                                         33  G    TiN (1) --      --    --      P + L (U)                                                                           P + L (I) + P                                                                         Al.sub.2 O.sub.3                                                              (2)  TiN (1)                                                    (2)   (2)                              34       TiCN (1)                                                                              TiCO (1)                                                                              --    --      P + L --      --   --                                                         (I) + P (3)                            35  H    TiCNO (0.5)                                                                           TiC (1) --    --      TiCN (1)                                                                            L (B) + P (3)                                                                         TiC                                                                                P + L                                                                         (I) + P                                                                       (3)                 36  I    TiCN (1)                                                                              TiN (1) --    --      P + L (U)                                                                           P + L (U) (2)                                                                         TiCN Al.sub.2                                                                      O.sub.3 (2)                                                (2)           (0.5)                    37  J    TiCN (1)                                                                              TiC (1.5)                                                                             TiN (0.5)                                                                           --      L (B) + P                                                                           TiN (1) --   --                                                         (3)                                    38       TiC (0.5)                                                                             --      --    --      P + L TiN     --   --                                                         (I) + P (5)                                                                         (0.5)                            39  K    TiCN (1)                                                                              --      --    --      P +   P + L (I) + P                                                                         L    --                                                         L (U) (2)                                                                           (2)     (B) + P                                                                       (2)                      40  L    TiN (1) TiC (1) TiCO (0.5)                                                                          Al.sub.2 O.sub.3 (2)                                                                  TiCN (0.5)                                                                          P + L (U) (2)                                                                         --   --                  __________________________________________________________________________     Note:                                                                         P = Granular crystal structure; L = Longitudinal growth crystal structure     U = Upper layer; I = Intermediate layer; B = Lower layer                 

                                      TABLE 15                                    __________________________________________________________________________    Classi-                                                                            Substrate                                                                          Composition and average thickness of the hard covering layer                  (μm)                                                             fiction                                                                            number                                                                             First layer                                                                          Second layer                                                                         Third layer                                                                         Fourth layer                                                                         Fifth layer                              __________________________________________________________________________    Prior art cutting tool covering                                               27   G    TiN (1)                                                                              TiCN (4)                                                                             Al.sub.2 O.sub.3 (2)                                                                TiN (1)                                                                              --                                       28        TiCN (1)                                                                             TiCO (0.5)                                                                           TiCN (3)                                                                            --     --                                       29   H    TiCNO (0.5)                                                                          TiC (1)                                                                              TiCN (4)                                                                            TiC (0.5)                                                                            TiCN (3)                                 30        TiCN (1)                                                                             TiN (1)                                                                              TiCN (4)                                                                            TiCNO (0.5)                                                                          Al.sub.2 O.sub.3 (2)                     31   I    TiCN (6)                                                                             TiN (1)                                                                              --    --     --                                       32        TiC (0.5)                                                                            TiCN (5)                                                                             TiN (0.5)                                                                           --     --                                       33   J    TiCN (1)                                                                             TiC (1)                                                                              TiCN (6)                                                                            --     --                                       34        TiN (1)                                                                              TiC (1)                                                                              TiCO (0.5)                                                                          Al.sub.2 O.sub.3 (2)                                                                 TiCN (3)                                 __________________________________________________________________________

                                      TABLE 16                                    __________________________________________________________________________    Amount of flank wear (mm)                                                     Classi-                                                                            Cutting                                                                              Cutting material                                                                       Classi-                                                                            Amount of flank wear (mm)                           fication                                                                           material: steel                                                                      cast-iron                                                                              fication                                                                           Cutting material: steel                                                                  Cutting material: cast-iron              __________________________________________________________________________    Cutting tool covering of                                                                           Prior art cutting                                        the present invention                                                                              tool covering                                            33   0.15   0.17     27   6 min. tool life due to                                                                  7 min. tool life due to                                            chipping   chipping                                 34   0.20   0.21     28   9 min. tool life due to                                                                  9 min. tool life due to                                            chipping   chipping                                 35   0.13   0.17     29   5 min. tool life due to                                                                  3 min. tool life due to                                            chipping   chipping                                 36   0.18   0.16     30   4 min. tool life due to                                                                  6 min. tool life due to                                            chipping   chipping                                 37   0.15   0.13     31   9 min. tool life due to                                                                  8 min. tool life due to                                            chipping   chipping                                 38   0.16   0.17     32   10 min. tool life due to                                                                 9 min. tool life due to                                            chipping   chipping                                 39   0.13   0.16     33   6 min. tool life due to                                                                  4 min. tool life due to                                            chipping   chipping                                 40   0.19   0.17     34   4 min. tool life due to                                                                  3 min. tool life due to                                            chipping   chipping                                 __________________________________________________________________________

What is claimed is:
 1. A process for producing a cermet cutting toolcomprising the steps of:a) preparing a substrate, having a surface, fromtitanium carbo-nitride based cermet; b) forming a coating layer onto thesurface of said substrate using a first chemical vapor deposition and asecond chemical vapor deposition; c) wherein said first chemical vapordeposition is performed using a reaction gas comprised of 1-5% of TiCl₄,0.1-1% of CH₃ CN, and 0-25% of N₂, with the remaining portion being H₂,at a reaction temperature of 800°-900° C. and a reaction pressure of30-200 Torr, so as to form at least one titanium carbo-nitride layerhaving a longitudinal growth crystal structure in which crystal grainsare elongated along a direction perpendicular to the surface of saidsubstrate; d) said second chemical vapor deposition is performed at areaction temperature of 950°-1050° C. so as to form at least onegranular crystal layer having a granular crystal structure, each saidgranular crystal layer being formed from a compound selected from thegroup consisting of titanium carbide (TiC), titanium nitride (TiN),titanium carbo-oxy-nitride (TiCNO), and aluminum oxide (Al₂ O₃); and e)said coating layer is formed by said first chemical vapor deposition andsaid second chemical vapor deposition to comprise at least one of thefollowing crystal structures:i) a crystal structure which changes fromsaid granular crystal structure to said longitudinal growth crystalstructure in going from said substrate to the surface of said coatinglayer; ii) a crystal structure which changes from said granular crystalstructure to said longitudinal growth crystal structure and back to saidgranular crystal structure in going from said substrate to the surfaceof said coating layer; and iii) a crystal structure which changes fromsaid longitudinal growth crystal structure to said granular crystalstructure in going from said substrate to the surface of said coatinglayer.
 2. A process for producing a cermet cutting tool according toclaim 1, wherein said coating layer is formed to have an averagethickness of 0.5-20 μm.
 3. A process for producing a cermet cutting toolaccording to claim 1, wherein said cermet substrate comprises:7- 95% byweight of a solid solution of carbo-nitride represented by the formula(Ti_(x) M_(y))(C_(u) N_(v)), wherein M is at least one metal selectedfrom the group consisting of transition metals in groups IV_(A), V_(A),and VI_(A) of the Periodic Table with the exception of titanium, and x,y, u, and v are molar ratios which satisfy the relations x+y=1, u+v=1,0.5≦x≦0.95, 0.05≦y≦0.5, 0.2≦u≦0.8, and 0.2≦v≦0.8, as a dispersedphase-constituting component; and at least one metal selected from thegroup consisting of cobalt and nickel as a binder phase-constitutingcomponent.
 4. A process for producing a cermet cutting tool according toclaim 3, wherein said binder phase-constituting component is moreconcentrated near the surface of said cermet substrate.